The present invention relates to an image forming apparatus, in particular, a copying machine, a printer, a facsimile machine, or the like, which forms an image with the use of an electrophotographic image forming method.
An electrophotographic process makes it possible to instantly form an image with high quality and durability. Thus, its usage did not remain in the field of a copying machine; it has come to be widely used not only in the field of a copying machine, but also in the fields of various printers and facsimile machines.
In principle, an electrophotographic process comprises two distinctive processes: an actual image formation process, and an initialization process. The actual image formation process comprises: uniform charging of a photoconductive member; formation of an electrostatic latent image through the exposure of the charged photoconductive member to an optical image in accordance with an original; development of the latent image with the use of toner; transferring of the toner image onto recording medium such as a piece of paper (or sometimes intermediary transfer medium); and fixation of the toner image, whereas the initialization process is a process for removing the toner particles and electrical charge remaining on the peripheral surface of the photoconductive member, in order to repeatedly use the photoconductive member. Further, according to some reports, in order to stabilize the potential level of a photoconductive member at an early stage of the charging process, an auxiliary charging device is disposed on the upstream side of the charging device, in terms of the moving direction of the peripheral surface of the photoconductive member, more specifically, between the cleaning means and charging means.
The nucleus of an electrophotographic image forming method is a photoconductive member which uses photoconductive substance. In recent years, a photoconductive member which uses electrically conductive organic substance has been developed. Electrically conductive organic substance has some advantages over electrically conductive inorganic substance; for example, it is environmentally harmless, and easy to form into film.
In an electrophotographic process, a photoconductive member is gradually shaved or scratched due to the friction which occurs during the development, transfer, and/or cleaning. Thus, eventually, the thickness of the charge retaining capacity of the outermost layer (film) of the photoconductive member is reduced, reducing thereby the charge retaining capacity of the photoconductive member to a point at which the image forming apparatus employing this photoconductive member begins to form unsatisfactory images, that is, the images the quality of which does not meet a predetermined requirement; in other words, the photoconductive member reaches the end of its service life, and must be replaced with a new one at this point.
It is true that an organic photoconductive members of the current generation is at a highly advanced level due to the recent developments in the field of a photoconductive member. However, the materials for the charge transfer layer, or the outermost layer, of a photoconductive member are still polycarbonate, vinyl polymer, polyester, and the like, which cannot be said to be sufficiently resistant to shaving for the photoconductive member to be satisfactorily used within an electrophotographic image forming apparatus. Thus, the amount of the portion of the charge transfer layer shaved away by the friction, and the number of scars created in the surface of the charge transfer layer by the friction, relatively quickly increase, shortening the service life of a photoconductive member. In other words, the service life of an organic photoconductive member is relatively short, expiring after outputting approximately 50,000 copies.
In comparison, a photoconductive member, the main constituent of which is non-crystal silicon, and which is commonly called an amorphous photoconductive member, has come into use in recent years. The surface layer of this type of photoconductive member is hard, and therefore, is highly resistant to shaving, affording an amorphous photoconductive member an image output exceeding 50,000. Further, referring to FIG. 9, in terms of the relationship (E-V property) between the amount of the drop in the surface potential level of an amorphous photoconductive member and the amount of exposure light, an organic photoconductive member is nonlinear, whereas an amorphous photoconductive member is virtually linear, which in this case is superior. For this reason, an amorphous photoconductive member is characterized in that the difference in diameter among the discrete dots resulting from the use of an amorphous photoconductive is smaller relative to the difference in latent image contrast. Further, the specific inductive capacity of an organic photoconductive member is 2-3, whereas the specific inductive capacity of the amorphous photoconductive member is approximately 10, which is relatively large. Therefore, a toner image formed by developing an electrostatic latent image formed on an amorphous photoconductive member is superior in the development of the smallest picture elements of an image, which is common knowledge. Thus, an amorphous photoconductive member is widely used in the field of a high speed image forming apparatus capable of forming high quality images.
Also in recent years, in order to obtain images of higher quality, to store or freely edit the inputted image formation data, or the like purposes, digitization of an image formation process has been rapidly progressing. Thus, even in the field of an amorphous photoconductive member, the materials suitable for digitization have been developed, some of them having been already put to practical use.
An amorphous photoconductive member, however, is greater in specific inductive capacity and electrostatic capacity than an organic photoconductive member. Thus, in order to charge an amorphous photoconductive member to a potential level high enough to form a satisfactory image using a corona discharge type charging method, a large amount of current is necessary to trigger electrical discharge to the photoconductive member.
Thus, when a charging method based on electrical discharge is used as a method for charging an amorphous photoconductive drum, a large amount of the byproducts of electrical discharge, for example, ozone, NOx, and the like, is likely to adhere to the peripheral surface of the amorphous photoconductive drum, reducing the electrical resistance of its peripheral surface, which in turn disturbs a latent image formed on the peripheral surface of the amorphous photoconductive drum, in particular in a high temperature/high humidity environment in which the surface resistance reduces. This disturbance of a latent image has a blurring effect, resulting in the formation of a defective image; areas of an image made up of discrete dots become blurred, making the areas look like flowing water.
For the reason given above, an amorphous photoconductive member, which normally is chargeable to the positive polarity, is more desirable as a photoconductive member than an amorphous photoconductive member, which normally is chargeable to the negatively polarity and therefore, produces a larger amount of the byproducts of electrical discharge, such as ozone, than the former.
There are two types of developing methods for developing an electrostatic latent image formed by exposing the peripheral surface of a photoconductive member charged to its natural polarity and a predetermined potential level, to an optical image irradiated in response to electrical signals obtained by processing the image formation data into optional toner reproduction patterns. One is a reversal developing method, in which toner which is the same in polarity as the polarity to which a photoconductive member is charged is used, and the other is a normal developing method, in which a reversal image exposure process is used.
Based on the above described knowledge and problems, we, the inventors of the present invention, decided to wrestle with the task of developing an image forming apparatus which was durable, capable of forming high quality images, smaller in the amount of the byproducts resulting from electrical discharge, and superior in terms of the prevention of the formation of blurred images (images suffering from appearance of flowing water) which were likely to be formed in a high temperature/high humidity (H/H) environment. As for the photoconductive member, because of the above described difference in the byproducts resulting from electrical discharge, we decided to use such an amorphous photoconductive member that is positively chargeable, durable, and capable of bearing a high quality latent image. As for the toner, we decided to use negatively chargeable toner, for which a wider selection of materials are available in terms of charge polarity. As for the exposing method, a background image exposing method (which hereinafter will be referred to as BAE method), that is, an exposing method which exposes the areas of the peripheral surface of a photoconductive member, which correspond to the non-image areas (background areas) of an intended image. As for the charging method, we decided to employ a corona discharge type charging method, which is capable of positively charging an amorphous photoconductive member so that a high quality latent image can be formed and developed, and the amount by which the byproducts generated by electrical discharge, such as ozone, is smaller.
First, in order to improve the controlling method to be used in the image forming apparatus for controlling the process for charging a photoconductive member, we studied the charging process controlling method proposed in Japanese Laid-open Patent Application 11-190922, in which essential control was carried out in the initial stage of a charging process.
More specifically, according to this patent application, in order to erase the hysteresis on a photoconductive drum, the photoconductive drum was exposed by an optical charge removing means immediately after the photoconductive drum begins to be rotated. Then, the charging of the photoconductive drum by a charging means is started. In order to ensure that the potential level of the photoconductive drum converged to a potential level equal to the potential level corresponding to a non-image area, the photoconductive drum was exposed to a proper amount of light for effecting the potential level corresponding to a non-image area (which hereinafter may be referred to as non-image area potential level), by the exposing means, in the early stage of the charging process, for a predetermined length of time which was set with the provision of some margin, in consideration of the fluctuation in the voltage applied to trigger electrical discharge to charge the photoconductive drum, the variation in the startup time of the exposing means, the fluctuation of the rotational speed of the photoconductive drum, the variation in the timing with which voltage is applied to the charging means, and the like factors. As a result, however, the following problems occurred.
(1) During the first rotational cycle of the photoconductive drum, the photoconductive drum was less uniformly charged, by a drastic margin, than during the second rotational cycle of the photoconductive drum and thereafter.
(2) Images suffering from ghosts were produced, the locations of which corresponded to the non-charged regions of the photoconductive drum (the region, which was cleared of electrical charge by the charge removing means, but was not charged by the charging device), and the region of the photoconductive drum, the location of which corresponds to the period in which the photoconductive drum was exposed to the optical image to reduce the potential level of the region of the photoconductive drum to the non-image potential level.
In the case of the BAE method employed by the present invention, a latent image is normally developed, in other words, toner is adhered to the areas of the photoconductive drum 1 with electrical charge (potential level of Vd). Therefore, the deviation in development contrast (difference in potential level between the development voltage and the area of photoconductive drum to which toner is adhered: Vcont) straightforwardly manifests as image density deviation. A ghost potential level, that is, the manifestation of the memory generated in the aforementioned non-charged region as the aforementioned non-charged region is exposed by the exposing means, is lower than a potential level, to which the photoconductive drum is charged by the charging means during the second rotational cycle of the photoconductive drum and thereafter. Therefore, a resultant image suffers from a narrow rectangular negative ghost, which extends in the direction perpendicular to the recording medium conveyance direction.
Further, even when correcting, in order to realize proper latent image contrast, the amount of the exposure light for latent image formation, corrections are made based on the potential level of the non-charged region of the photoconductive drum, which is detected by the potential level detecting means to confirm the accuracy of the potential level, to which the potential level of the photoconductive drum will drop as the photoconductive drum is exposed, in other words, based on the potential level of the region in which an optical memory has already been generated by the excessive amount of charge removing light irradiated by the charge removing means, and also by the exposing means. Therefore, it is impossible to accurately calculate a compensatory amount for realizing the proper potential level VI (potential level resulting from maximum exposure). As a result, defective images were produced.
We also studied a charging method, such as the one disclosed in Japanese Patent Application Publication 10-123802, which employed an auxiliary charging device, as a countermeasure for the above described problem. However, the provision of an auxiliary charging device increases product cost. Also, usage of corona type charging device as an auxiliary charging device increases the ozone concentration. Further, in the case of an electrophotographic image formation method employing an amorphous photoconductive member, the provision of an auxiliary charging device adds to the number of factors which effect defective images, more specifically, partially or totally blurred images giving an appearance of flowing water. Thus, in order to provide an image forming apparatus capable of reliably forming high quality images, a charging method capable of eliminating the above described problems without the provision of an auxiliary charging device has been desired.
Thus, the primary object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of image defects traceable to nonuniformity in potential level.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.